[Fix #6778] Fix a false positive in Style/HashSyntax cop

[tatsuyafw]

Fixes #6778

The Style/HashSyntax cop would register an offense when EnforcedStyle was ruby19_no_mixed_keys and a hash key was an interpolated string like the following code:

{"#{foo}": 1}

This change fixes that.

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[bbatsov]

Thanks!